In the formation of integrated circuits (IC), thin films containing metal and metalloid elements are often deposited upon the surface of a semiconductor substrate to provide conductive and ohmic contacts in the circuits and between the various devices of an IC. For example, a thin film of a desired metal might be applied to the exposed surface of a contact or via hole on a semiconductor substrate. The film, passing through the insulative layers on the substrate, provides plugs of conductive material for the purpose of making interconnections across the insulating layers.
One well known process for depositing a thin metal film is chemical vapor deposition (CVD). In CVD, a thin film is deposited using chemical reactions between various deposition or reactant gases at the surface of the substrate. Reactant gases are pumped into proximity to a substrate inside a reaction chamber, and the gases subsequently react at the substrate surface resulting in one or more reaction by-products which form a film on the exposed substrate surface.
A thin metal film deposited by chemical vapor deposition on a substrate has a smooth, mirror-like surface morphology. As more film is deposited, however, the surface morphology of the film becomes rough and hazy in direct proportion to the film thickness. A rough surface is undesirable because it results in electromigration, whereby metal atoms are transported along grain boundaries driven by the force exerted by flowing electrons under high current densities. As a result, voids form at one end and extrusions form at the other end of the metal lines. This leads to an increased probability of circuit failure in the area of the voids.
Where a copper film is deposited, in addition to a smooth surface morphology, it is desirable for commercial production of semiconductor substrates that the films have a resistivity that is close to a bulk resistivity of 1.68 .mu.ohms-cm, which is the resistivity of pure copper metal. Copper films with a resistivity higher than about 2 .mu.ohms-cm have less viability as a manufacturing technology since other metals with similar high resistivities can replace copper. Factors that may influence film resistivity include film thickness, density, purity, and grain size.
Accordingly, there is provided a method of producing substrates coated with thick metal films that have a smooth surface morphology and low resistance.